The present invention relates to a method for correcting nonlinearities of a radio power amplifier.
The invention finds applications in radio transmitters, in particular mobile radio communications stations.
Mobile radio communications digital systems are increasingly making use of radio modulations with non-constant envelope in order to improve the spectral efficiency of the system. These modulations with nonconstant envelope comprise, in particular, quadrature phase shift keying modulations (QPSK, OQPSK or .pi./4-QPSK), or quadrature amplitude modulations (n-QAM).
For a given data rate, these modulations have the advantage of requiring a smaller frequency bandwidth than the modulations with constant envelope which are frequently used, such as GMSK modulation. The downside of this advantage is that modulations with nonconstant envelope require a very linear transmission system in order to avoid spectral broadening due to nonlinearities. The critical point of this system is usually the power amplifier of the transmitter. In the case of a mobile radio communications station, for which the electrical power consumption must be minimized, the use of a class A linear amplifier is not generally acceptable because of its inadequate efficiency. A more appropriate solution consists in using a nonlinear amplifier with high efficiency, combined with a linearization technique.
The technique of adaptive linearization using predistortion is an amplifier linearization technique which can be used for this purpose. This technique is described in the article "Adaptive Linearisation Using Predistortion" by M. Faulkner et al. (Proc. of the 40th IEEE Veh. Tech. Conf. 1990, pages 35-40), and in U.S. Pat. No. 5,093,637. The technique of adaptive linearization using predistortion consists in applying a predistortion table to the baseband complex signal. This table is calculated digitally by comparing the baseband signal and a demodulated signal obtained from the amplified radio signal. This calculation consists in modifying the predistortion table in order to obtain a table which corrects the distortions of the transmission system. After an initial adaptation period, the predistortion table corrects the nonlinearities of the amplifier.
The predistortion algorithms employed are generally based on the assumption that the distortion introduced by the power amplifier depends only on the modulus of the complex baseband signal, and not on its phase (AM-AM and AM-PM distortions only) . Consequently, predistortion functions are chosen which depend only on the modulus of the signal. Nevertheless, some distortions may also be introduced by the modulator used to transpose the signal around the carrier frequency, as well as by the demodulator producing the demodulated signal which is useful in the adaptation periods. The modulator and the demodulator may have balance and/or quadrature defects which, besides introducing PM-AM and PM-PM distortions, risk making the adaptation algorithm converge to an inappropriate predistortion table. The demodulated signal may further have an offset with respect to the baseband adaptation signal. Another problem which may interfere with the convergence of the predistortion algorithm is the presence of noise in the demodulated signal.
One object of the present invention is to provide an efficient linearization technique which is affected less by the problems above.